The technique of nanosecond fluorometry will be employed to monitor the conformational changes of the allosteric enzyme phosphorylase b which are induced by the substrate glucose-1-phosphate and the allosteric modifier AMP. The fluorescent probe 1-anilino-8-naphthalene sulfonate (ANS), when bound by the enzyme, behaves as two fluorescent components with fluorescent decay times close to 19 nanosecond and 8 nanoseconds. The addition of AMP or glucose-1-phosphate results in the loss of the 19 nanosecond component. These results have been interpreted in terms of two binding sites for ANS which correspond to different microenvironments. The conformational changes associated with binding of substrate or activator result in the elimination of the site responsible for the 19 nanosecond decay time. If the rotational relaxation time is determined from the time decay of anisotropy, it is found that the relaxation time decreases in the presence of substrate or modifier. In future work it is planned to correlate these changes with quantitative measurements of the binding of AMP in order to establish whether phosphorylase b conforms to any of the proposed allosteric models.